1. Field of the Invention
The present invention is related to the control of power limits at renewable energy sites such as solar, wind, and tidal energy sites. More particularly, the present invention relates to renewable energy site power limit control that addresses conditions of plant saturation and loop delay.
2. Description of Related Art
Renewable energy sites are typically composed of multiple power conversion devices connected in parallel generating fixed frequency AC power to a grid. The devices are typically AC-AC or DC-AC inverters. Inverters are designed to extract maximum power from the renewable power supply, subject to a real power limit reference and often, a reactive power command.
A simple and common scenario is to configure inverters individually with the same fixed power limit and reactive power reference and allow them to operate independently to deliver power to the grid. The system is connected in terms of power output, but not control. However, this scenario tends to underutilize the total available power when it varies from inverter to inverter, and cannot regulate total site output power. Furthermore, such a system cannot regulate power at a point on the grid separated from the collective inverter outputs by grid impedance, especially when there are disturbances in grid voltage and load.
More recently, site controllers have been employed to reduce these problems by measuring site total power feedback to actively control site total power. As shown in FIG. 1, the site control loop 10 consists of commands 23 from the controller 12 to the inverters 18 and feedbacks 21 from the inverters 18 and/or a utility meter 14 back to the site controller 12. The inverters 18 are connected to a power source 16 such as a photovoltaic (PV) module, and a step-up transformer 20 may intervene between the inverters 18 and the power meter 14.
The site controller typically generates the site-wide inverter command by feeding the site power error between the feedback and reference through a PI or I controller to generate a site-wide power command and then divides that equally among inverters to generate a command to send to individual inverters. The inverter power controllers are typically much faster than the site control loop. Therefore it is important to implement as much control functionality by the inverter itself, if possible.
However, traditional controllers can be improved by an effective means of handling non-ideal plant saturation and control loop delay, which cause many problems. The methods, systems, computer program products, and devices of the present invention are designed to overcome the following challenges of non-ideal operation encountered when implementing site power limit control: 1) Poor utilization of available power near limits of inverter saturation when available power is unequally distributed among inverters. The plant can appear quite nonlinear in this situation and performance suffers if appropriate anti-windup and saturation-dependent integration techniques are not used; and 2) Degraded controller dynamic response due to significant delay in the communications channels used to implement the control loop. Loop delay can result in unnecessary integrator windup during reference slewing and general instability. It can also result in large site power overshoot in the case of a power surge if power limit commands are not distributed to inverters appropriately.
The first problem is that of quickly maximizing utilization of available power when it is unequally distributed among inverters. This might occur, for example, at a PV site on a cloudy day when the PV array is partially shaded. In this case, the total site output drops below the site power reference. To compensate, the present invention increases the site-wide inverter power limit commands by incrementing an integrator which changes according to the error between the site power limit reference and site power feedback. The change in the integrator is chosen to take into account the number of inverters which are capable of producing more power. Furthermore, the present invention provides anti-windup techniques to prevent the integrator from increasing while no inverter is capable of increasing power output.
The second problem is that of handling significant site control loop delay. This delay is due to site controller task time and communication latency. This delay greatly restricts controller bandwidth. To reduce dependence on loop delay, exemplary embodiments of the present invention employ a feed-forward term which provides a fast transient response independent of feedback loop delay. The feed-forward term sums with an error term which maintains zero steady state error. Loop delay causes undesired integrator windup when the power limit reference changes. In the present invention the integrator error term is modified to compensate for this effect of loop delay. Another side effect of loop delay is that instantaneous site power surges cannot be corrected in less time than the loop delay. Often, the utility imposes restrictions such as acceptable site power overshoot or site power positive slew rate. At a PV site, for example, available solar irradiance may change very quickly as clouds pass overhead. Although there is nothing that can be done to prevent a sudden drop in available power, a sudden increase in available power can be prevented, even if the site control loop is not quick enough to react to it. The present invention addresses this issue by constraining each inverter power limit to be no larger than a certain margin higher than the inverter output power. This margin is the effective maximum real power overshoot that could be expected to an instantaneous surge in available inverter power. Incidentally, the margin also has the effect of slew limiting inverter power up, which is typically desired.
Various power controllers exist, such as those described in U.S. Pat. Nos. 8,260,469; 7,923,862; 7,890,217; and 6,512,966; as well as in U.S. Patent Application No. 2003/0006613 (which patents and publication are hereby incorporated by reference herein in their entireties), suffer from one or more of the limitations described herein. There is a need for improved renewable energy plant control that addresses issues of site power saturation and loop delay so that dynamic performance can be significantly improved.